(1) Field of the Invention
The present invention relates to a polyester resin for a toner usable for developing an electrostatic charged image and a magnetic latent image in electrophotography, electrostatic recording, electrostatic printing and the like.
(2) Description of the Related Art
For forming an image in electrophotography, electrostatic printing or the like, a method is adopted in which an image to be recorded is converted to an electronic latent image such as an electrostatic charged image or an electroconductive image on the surface of an electrostatic recording material, the latent image is developed and formed with a charged toner, and the formed toner image on the surface of the electrostatic recording material is transferred and fixed on a paper sheet or a recording film. This image-forming method is advantageous in that the copying operation can be performed at a high speed, the fixing stability of the image on the surface of the recording material is good, and the apparatus used can be manipulated very easily. Accordingly, this image-forming method is widely utilized in the fields of copying machines, laser printers and the like.
For example, for a formation of images by electrophotography, the surface of a roller composed of a photoconductive photosensitive material is subjected to a charging treatment, an electrostatic latent image formed by light exposure to rays reflected from the surface of an original is formed with a toner, the obtained toner image is transferred to a paper sheet or the like, and the transferred toner image is fixed to the paper sheet by compression under heating.
As the binder resin for the toner, there are used a styrene/acrylic resin, a polyester resin, an epoxy resin and the like, but recently a polyester resin has attracted attention as a binder resin having an excellent fixing property, giving a good image, and having a good resistance to plasticizers which are conventionally used for polyvinyl chloride and other resins.
As important properties required for the toner, there can be mentioned the fixing property and the non-offset property. A process for providing a toner having an excellent fixing property and non-offset property, by giving a crosslinked structure to a binder resin by using a trifunctional or higher polyfunctional monomer, has been proposed. A typical instance of the polyester resin for a toner having a crosslinked structure is disclosed in the specification of U.S. Pat. No. Re. 31,072.
This polyester resin having the crosslinked structure is inevitably prepared through a crosslinking reaction. The crosslinking reaction is different from the polycondensation reaction for a usual linear polyester resin in that the crosslinking reaction is abruptly advanced and a gel is readily formed. Accordingly, the control of the reaction is a very serious problem. In the specification of U.S. Pat. No. 4,849,495, the present inventors have proposed a process for the preparation of a crosslinked polyester in which the polycondensation reaction is carried out under a high vacuum of 150 Torr or less, the vacuum degree is gradually reduced from the point of a start of the crosslinking reaction, and the advance of the crosslinking reaction is thus controlled. Nevertheless, it has been found that, when this preparation process is conducted on an industrial scale, the gelation control sometimes becomes difficult according to the composition of monomers. Namely, it has been found that, in the industrial preparation of a crosslinked polyester, the ease of the control of the gelation depends greatly on the composition of monomers.